Applied Spectroscopy

The purpose of this study is to assess the feasibility of using breath ammonia analysis based on off-axis cavity-enhanced absorption spectroscopy (OA-CEAS) with an external-cavity diode laser (ECL) for noninvasive, real-time diagnosis of Helicobacter pylori (HP) infection. Analyses are performed for the breath of 15 healthy volunteers, and eight children and 19 adults with HP infection. The range of ammonia levels for healthy participants is determined to be between 178 and 610 ppb, whereas the ranges for child and adult patients with HP infection are measured to be 457–2470 ppb and 450–2990 ppb, respectively. The ammonia concentrations for patients with HP infection are significantly higher than the concentrations for healthy volunteers. However, no sharp boundary between the ammonia concentrations in the breath of patients with HP infection and healthy volunteers is observed. No correlation between breath ammonia and either body mass index (BMI) or age is found. The reported results suggest that our breath ammonia measurement system has the potential for future use in easy, noninvasive diagnosis of HP infection.

An investigation of the infrared (IR) spectra of polyoxymethylene (POM) mold plates was undertaken to determine the sub-micron-scale morphology and molecular orientation. The nest-structured cells concerned with the orientation were observed from scanning electron microscope (SEM) measurements with the aid of Raman spectroscopy. The intensity of the anomalous IR reflectance peak of the C–O stretching A₂ mode depends on the widths of the POM layers in the SEM image along the orientation direction. The results suggest that the spectral features originate from the Berreman effect of the bulk polaritons and the radiative surface polaritons. Moreover, the IR spectra of certain treated samples suggest that enhancement of the electromagnetic fields from the gap modes and transition dipole–dipole coupling influence the spectral shapes.

A new method of performing pump–probe experiments is proposed and experimentally demonstrated by a proof of concept on the millisecond scale. The idea behind this method is to measure the total probe intensity arising from several time points as a group, instead of measuring each time separately. These measurements are multiplexes that are then transformed into the true signal via multiplication with a binary Hadamard S matrix. Each group of probe pulses is determined by using the pattern of a row of the Hadamard S matrix and the experiment is completed by rotating this pattern by one step for each sample excitation until the original pattern is again produced. Thus to measure n time points, n excitation events are needed and n probe patterns each taken from the n × n S matrix. The time resolution is determined by the shortest time between the probe pulses. In principle, this method could be used over all timescales, instead of the conventional pump–probe method which uses delay lines for picosecond and faster time resolution, or fast detectors and oscilloscopes on longer timescales. This new method is particularly suitable for situations where the probe intensity is weak and/or the detector is noisy. When the detector is noisy, there is in principle a signal to noise advantage over conventional pump–probe methods.

Femtosecond laser-induced breakdown spectroscopy (LIBS), a powerful and versatile multi-elemental analysis technique, is employed for quantification of noble metal catalysts (Pt–Pd) loaded on cordierite matrix. The laser-induced plasma was generated on the sample surface using 1 mJ energy from a 50 fs, 1 kHz, Ti–Sa laser system. The spectral emission of the cordierite matrix and noble metals are systematically studied to identify 270.23 nm and 340.42 nm lines for quantitative estimation of Pt and Pd, respectively. Quantification of the LIBS signal is further aided by measurement of excitation temperature and electron density of the laser-ablated plasma. Time-resolved LIBS is used under optimized conditions to evaluate the analytical predictive ability of the technique. Calibration curves for Pt and Pd exhibit good linearity. The limit of detection for Pt and Pd is estimated to be 55 µg/g and 17 µg/g, respectively, of the cordierite matrix.

The objective of this research was to develop a rapid noninvasive method for quantitative and qualitative determination of chilled pork spoilage. Microbiological, physicochemical, and organoleptic characteristics such as the total viable count (TVC), Pseudomonas spp., total volatile basic-nitrogen (TVB-N), pH value, and color parameter L* were determined to appraise pork quality. The hyperspectral scattering characteristics from 54 meat samples were fitted by four-parameter modified Gompertz function accurately. Support vector machines (SVM) was applied to establish quantitative prediction model between scattering fitting parameters and reference values. In addition, partial least squares discriminant analysis (PLS-DA) and Bayesian analysis were utilized as supervised and unsupervised techniques for the qualitative identification of meat spoilage. All stored chilled meat samples were classified into three grades: “fresh,” “semi-fresh,” and “spoiled.” Bayesian classification model was superior to PLS-DA with overall classification accuracy of 92.86%. The results demonstrated that hyperspectral scattering technique combined with SVM and Bayesian possessed a powerful capability for meat spoilage assessment rapidly and noninvasively.

Predicting forage feed value is a vital part of estimating ruminant performances. Most near-infrared (NIR) reflectance calibration models have been developed on oven-dried green forages, but preserved forages such as hays or silages are a significant part of real-world farm practice. Fresh and preserved forages give largely similar fodder, but drying or ensiling processes could modify preserved forage spectra which would make the oven-dried green forage model unsuitable to use on preserved forage samples. The aim of this study was to monitor the performance of oven-dried green forage calibration models on a set of hay and silage to predict their nutritive value. Local and global approaches were tested and 1025 green permanent grassland forages, 46 types of hay, and 27 types of silage were used. The samples were scanned by NIR spectroscopy and analyzed for nitrogen, neutral detergent fiber, acid detergent fiber, and pepsin–cellulase dry matter digestibility (PCDMD). Local and global calibrations were developed on 975 oven-dried green forage spectra and tested on 50 samples of oven-dried green forages, 46 samples of hay, and 27 samples of silage. For oven-dried green forage and hay validation sets, Mahalanobis distance (H) between these samples and the calibration population center was lower than 3. No significant standard error of prediction differences was obtained when calibration models were applied to oven-dried green forage and hay validation sets. For silage, the H-distance was higher than 3, meaning that calibration models built from oven-dried green forages cannot be applied to silage samples. We conclude that local calibration outperforms global strategy on predicting the PCDMD of oven-dried green forages and hay.

The curing of coatings of two types of siloxane containing materials, silica gel and silsesquioxane, at a modest temperature (<280℃) was studied with in situ heating Fourier transform infrared spectroscopy (FT-IR) in combination with perturbation correlation moving window (PCMW) and two-dimensional correlation spectroscopy (2D-COS) analyses. The result revealed detailed structural evolution of these two different gels. When the silica gel was heated, (Si–O)₆ rings appeared from the random Si–O–Si network formed after sol gel reaction, followed by condensation of silanol groups. Upon further heating, the existing (Si–O)₄ rings were broken down and converted into (Si–O)₆ structures, and finally isolated silanols appeared. The transition from (Si–O)₄ rings to (Si–O)₆ rings was observed by IR and further confirmed with positron annihilation lifetime spectroscopy (PALS). In comparison, during the curing of hybrid silsesquioxane, the condensation of silanols happens immediately upon heating without the rearrangement of Si–O–Si network. Afterwards, the fraction of (Si–O)₆ ring structure increased. (Si–O)₄ structures exhibited higher stability in hybrid silsesquioxanes. In addition, the amount of silanols in silsesquioxane continued to reduce without the generation of isolated silanol in the end. The different curing behavior of silsesquioxanes from silica gel originates from the organic groups in silsesquioxanes, which lowers the cross-linking density and reduces the rigidity of siloxane network.

Raman spectra for various nitramine energetic compounds were investigated as a function of crystal size at the nanoscale regime. In the case of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), there was a linear relationship between intensity of Raman spectra and crystal size. Notably, the Raman modes between 120 cm⁻¹ and 220 cm⁻¹ were especially affected, and at the smallest crystal size, were completely eliminated. The Raman spectral intensity of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), like that of CL-20's, depended linearly on crystal size. The Raman spectral intensity of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), however, was not observably changed by crystal size. A non-nitramine explosive compound, 2,4,6-triamino-1,3,5- trinitrobenzene (TATB), was also investigated. Its spectral intensity was also found to correlate linearly with crystal size, although substantially less so than that of HMX and CL-20. To explain the observed trends, it is hypothesized that disordered molecular arrangement, originating from the crystal surface, may be responsible. In particular, it appears that the thickness of the disordered surface layer is dependent on molecular characteristics, including size and conformational flexibility. Furthermore, as the mean crystal size decreases, the volume fraction of disordered molecules within a specimen increases, consequently, weakening the Raman intensity. These results could have practical benefit for allowing the facile monitoring of crystal size during manufacturing. Finally, these findings could lead to deep insights into the general structure of the surface of crystals.

¹³C cross-polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) and µ-Raman spectroscopy were applied to characterize Sicilian amber samples. The main goal of this work was to supply a complete study of simetite, highlighting discriminating criteria useful to distinguish Sicilian amber from fossil resins from other regions and laying the foundations for building a spectroscopic database of Sicilian amber. With this aim, a private collection of unrefined simetite samples and fossil resins from the Baltic region and Dominican Republic was analyzed. Overall, the obtained spectra permitted simetite to be distinguished from the other resins. In addition, principal component analysis (PCA) was applied to the spectroscopic data, allowing the clustering of simetite samples with respect to the Baltic and Dominican samples and to group the simetite samples in two sets, depending on their maturity. Finally, the analysis of loadings allowed for a better understanding of the spectral features that mainly influenced the discriminating characteristics of the investigated ambers.

Brillouin spectroscopy is an emerging tool for microscopic optical imaging as it allows noninvasive assessment of viscoelastic properties of materials. The use of atomic–molecular absorption cells as ultra-narrow notch filters allows acquisition of Brillouin spectra from turbid samples despite their strong elastic scattering. However, such systems alter the shapes of the Brillouin lines, making the precise determination of the Brillouin shift difficult. In this report, we propose a simple method for analyzing the Brillouin spectrum using a customized least-square fitting algorithm. The absorption spectrum induced by the atomic–molecular cell was taken into consideration. The capability of the method is confirmed by processing experimental spectroscopic data from the pure water at different temperatures. The accuracy of the measurements of ±1 MHz spectral line shift is experimentally demonstrated.

This study focuses on the analysis of the optical emission response obtained by laser-induced breakdown spectroscopy from energetic nitro compounds in condensed phase sampled in atmospheres of variable composition. The influence of different background gases was evaluated from the characteristic emissions of the excited species coexisting in the plasma plume and conclusions concerning the main pathways involved in the generation of such emission species were extracted. Different reactive (O₂, N₂, H₂) and inert (Ar, He) gases were tested to establish the comparative emission features of organic compounds.

This manuscript describes a simple process for fabricating gold-based, multi-layered, surface-enhanced Raman scattering (SERS) substrates that can be applied to a variety of different nanostructures, while still providing multi-layer enhancement factors comparable to those previously achieved only with optimized silver/silver oxide/silver substrates. In particular, gold multi-layered substrates generated by atomic layer deposition (ALD) have been fabricated and characterized in terms of their optimal performance, revealing multi-layer enhancements of 2.3-fold per spacer layer applied. These substrates were fabricated using TiO₂ as the dielectric spacer material between adjacent gold layers, with ALD providing a conformal thin film with high surface coverage and low thickness. By varying the spacer layer thicknesses from sub-monolayer (non-contiguous) films through multiple TiO₂ layer thick films, the non-monotonic spacer layer thickness response has been elucidated, revealing the importance of thin, contiguous dielectric spacer layers for optimal enhancement. Furthermore, the extended shelf life of these gold multi-layered substrates was characterized, demonstrating usable lifetimes (i.e. following storage in ambient conditions) of greater than five months, with the further potential for simple limited electrochemical regeneration even after this time.

Waveguide-enhanced Raman spectroscopy (WERS) is emerging as an attractive alternative to plasmonic surface-enhanced Raman spectroscopy approaches as it can provide more reproducible quantitative spectra on a robust chip without the need for nanostructured plasmonic materials. Realizing portable WERS systems with high sensitivity using low-cost laser diodes and compact spectrometers requires a detailed analysis of the power budget from laser to spectrometer chip. In this paper, we describe theoretical optimization of planar waveguides for maximum Raman excitation efficiency, demonstrate WERS for toluene on a silicon process compatible high index contrast tantalum pentoxide waveguide, measure the absolute conversion efficiency from pump power to received power in an individual Raman line, and compare this with a power budget analysis of the complete system including collection with an optical fiber and interfacing to a compact spectrometer. Optimized 110 nm thick Ta₂O₅ waveguides on silica substrates excited at a wavelength of 637 nm are shown experimentally to yield overall system power conversion efficiency of ∼0.5 × 10⁻¹² from the pump power in the waveguide to the collected Raman power in the 1002 cm⁻¹ Raman line of toluene, in comparison with a calculated efficiency of 3.9 × 10⁻¹². Collection efficiency is dictated by the numerical and physical apertures of the spectral detection system but may be improved by further engineering the spatial and angular Raman scattering distributions.